WATER AND SOIL QUALITY MANAGEMENT IN PONDS AND OTHER MARICULTURE FACILITIES

Water properties and environmental parameters -affect the choice of an aquaculture site and the cultured species Reverse effect of Fish culture -oxygen consumption and metabolic byproducts such as ammonia and carbon dioxide- Water quality standards for aquaculture are presented in the following table-

PARAMETERCONCENTRATION (mg/L except for pH) Alkalinity Ammonia (NH3)<0.02 Ammonia (TAN)<1.0 Calcium4-160 Carbon dioxide0-10 Dissolved oxygen5 to saturation Hardness, Total Hydrogen sulfide<0.003 Iron<0.01 Magnesium<15 Manganese<0.01 Nitrogen gas<110% total gas pressure <103% as nitrogen gas Nitrite<0.1 Nitrate0-3 PCB’s<0.002 pH Potassium<5 Salinity15-35 ppt. Total suspended solids<80 Zinc<0.005

Temperature: Has greatest effect on fish - poikilotherms Temperature also affects oxygen solubility and causes interactions of several other water quality parameters Also influence physiological processes such as respiration rate, efficiency of feeding and assimilation, growth, behaviour and reproduction A temperature increase of 10°C will generally cause rate of chemical and biological reactions to double or triple

Because of this dissolved oxygen requirements are more critical in warm water than in cold water Optimum temperature varies with the species Within a species’ tolerable limits growth will increase with increasing temperature Fish -healthiest - optimum temperature

Salinity Salinity - measure of the concentration of dissolved ions in water expressed as parts per thousand (ppt) Seawater salinity - 33 to 37 ppt with an average of about 34 Estuarine and brackish waters - from full strength seawater to about 3 ppt. The composition and concentration of dissolved salts in the body fluids of fish and invertebrates must be maintained within fairly narrow limits to buffer against changes that can cause physiological disruptions

Fish maintain homeostasis through osmoregulation which is influenced by salinity of water Species tolerating wide rage of salinity - ‘Euryhaline’ Limited tolerance to salinity changes -‘stenohaline’ Euryhaline species (Asian sea bass) can be cultured in a broad range of salinity from fresh to seawater but stenohaline fish (cobia) can be cultured only in full strength seawater

Turbidity Turbidity -measure of light penetration in water It is produced by dissolved and suspended substances such as clay particles, humic substances, plankton, coloured compounds etc. Excessive turbidity can be troublesome in fish ponds and flow-through systems Turbidity by plankton -desirable since it enhances fish production Turbidity due to clay and other colloidal particles - undesirable since it can choke the gills of fish and shellfish; can also restrict the growth of phytoplankton Water turbidity in ponds - measured with the Secchi disc

Is a round disk having a diameter of 30 cm The disk is divided into quadrants, two opposite quadrants are painted white and the other two black The disc is attached to a rope or cable marked in increments A measurement is taken by lowering the disk into water body until it just disappears from sight The depth at which the disk disappears is the Secchi disc visibility usually expressed in centimeters Optimum Secchi disc visibility in extensive and semi-intensive ponds is 25 to 40 cm

Chemical variables Dissolved oxygen: Dissolved oxygen - controls the metabolism of fish and invertebrates Main source - dissolution from the atmosphere Phytoplankton and macrophytes - through photosynthesis Increase in temperature and salinity reduce the saturation point of DO in water Fish oxygen consumption rates - vary with water temperature, DO concentration, fish size, level of activity, time after feeding and other factors

Small fish consume more oxygen per unit weight than larger fish of the same species Oxygen consumption increases when fish are forced to exercise, and metabolic energy demands can cause oxygen consumption to double from one to six hours after feeding Warm water species tolerate lower DO concentration than cold water fish Warm water fish die after short term exposure to less than 0.3 mg/L DO To support life for several hours, a minimum of 1.0 mg/L is required, and 1.5 mg/L is required to support fish for several days Optimum DO levels should be above 5.0 mg/L

Total alkalinity: Total alkalinity is the total amount of titratable bases in water expressed as mg/L of equivalent calcium carbonate The principal ions contribute to alkalinity are carbonate and bicarbonate and, to a lesser degree, hydroxides, ammonium, borate, silicates and phosphates Alkalinity is a measure of pH buffering capacity or acid neutralizing capacity Natural seawaters have a mean total alkalinity of about 116 mg/L Alkalinity below 30 mg/L is considered poorly buffered against pH changes

pH: pH - indicator of hydrogen ion concentration in water The mean pH of ocean is about 8.3 and remains fairly constant because of the great buffering capacity of the oceans Carbon dioxide: The main source of carbon dioxide to water is by respiration of organisms and biological oxidation of organic matter Carbon dioxide is not particularly toxic to fish provided sufficient dissolved oxygen is available A CO₂ concentration of mg/L is recommended as maximum for fish culture

Ammonia: Ammonia exists in water in two states - ionized ammonia, also called the ammonium ion (NH4+) and un-ionized ammonia (NH3) The sum of two is called total ammonia or simply ammonia The toxicity of total ammonia depends on what fraction of the total is in the un- ionized form, since this form is by far the more toxic of the two Which fraction dominates depends on the pH, temperature and salinity of water; out of this water pH has the strongest influence At higher pH un-ionized ammonia dominates and hence more toxic A maximum concentration of 0.01mg/L is recommended for marine fish

Nitrite: Nitrite -toxic to fish Nitrite levels in fish ponds typically range from 0.05 to 5 mg/L, probably due to reduction of nitrate under anaerobic conditions The toxicity of nitrite is due to its effect on oxygen transport and tissue damage Fish deaths increase when low dissolved oxygen is coupled with higher nitrite concentrations DO levels should be higher than 6 mg/L when fish are affected by nitrite toxicity At higher levels of chloride the toxicity of nitrite is reduced. Hence nitrite toxicity in seawater is not as serious as in freshwater systems.

Nitrate: Nitrates are the least toxic of the inorganic nitrogenous compounds Nitrates are more of a problem in recirculatory systems and the problem is controlled with daily water exchanges Hydrogen sulphide: Hydrogen sulphide is generated by certain heterotrophic bacteria under anaerobic conditions The distribution of hydrogen sulphide and other sulphur species is regulated by water pH The un-ionized form accounts for about 99% of the total sulphide As the value of pH rises, the percent of hydrogen sulphide decreases Un-ionized hydrogen sulphide is toxic to fish and invertebrates at low concentrations. Therefore, detectable concentrations should be considered as hazardous